Such a device is known from publication no. EP 2 508 920 A1. As further described in this publication, the light emitter of the device emits light to a retroreflector reflecting the light back to the light receiver. Substantially no light stemming from any diffusing matter, such as white paper, or from any reflecting matter apart from the retroreflector shall be detected by the device. In order to reduce the detection of such diffused and reflected light to a large extent, the polarization unit of the device is equipped with two polarizers, wherein the emitted light is polarized by one polarizer and the light to be detected is polarized by the other polarizer. Both polarizers yield a differing polarization of the light passing through. The retroreflector causes a depolarization of the arriving light and emits it back in the same direction.
An application of the device is a detection of objects crossing the light path in between the retroreflector and the light receiver, in particular of objects comprising a section that is transparent or translucent in the visible light spectrum. The detection is based on an output signal generated by the device in dependence of light detected or not detected by the light receiver indicating the absence or presence of an object within a monitoring area.
The previously disclosed device, however, has a number of shortcomings. On the one hand, the polarization unit includes two different polarizers respectively accounting for a polarization of the outgoing light beam and a differing polarization of the incoming light beam. A twofold supply of those components represents a considerable cost factor, even more so when the components must match to a desired ultraviolet wavelength range. It also leads to a rather complex composition and manufacturing of the device. The provision of two polarizers arranged at different positions in the device can also negatively affect the detection reliability.